1. Technical Field of the Invention
The present invention relates generally to a common rail fuel injection system equipped with a fuel pressure sensor working to measure the pressure of fuel within a common rail and an electronic control unit working to monitor an output of the fuel pressure sensor to determine the pressure within the common rail, and more particularly to such a system designed to avoid an error in determining the pressure within the common rail.
2. Background Art
Japanese Patent First Publication No. 2002-276500 discloses one example of typical common rail fuel injection systems which includes a common rail in which fuel is accumulated under a given high pressure and fuel injectors working to inject the fuel in the common rail into the engine. The common rail has installed therein a fuel pressure sensor which measures the pressure of the fuel within the common rail. The system works to monitor an output of the fuel pressure sensor and control the quantity of fuel to be discharged from a high-pressure pump to the common rail to bring the pressure in the common rail into agreement with a target value.
The fuel pressure sensor is coupled with an electronic control unit through a harness. The electronic control unit is equipped with a power supply circuit designed to produce a constant power supply voltage from a storage battery and supply it to the fuel pressure sensor.
The fuel pressure sensor has installed therein a sensor element which is sensitive to the pressure in the common rail. The sensor element is made of a semiconductor designed to exhibit the piezoresistance effect in which the resistance value changes upon deformation thereof. Specifically, the sensor element has an electric resistivity which changes as a function of a change in the pressure in the common rail. This resistivity change is found by using voltages applied to and outputted from the sensor element. The voltage applied to the sensor element is the constant power supply voltage, as produced by the power supply circuit. The electronic control unit monitors the voltage outputted by the fuel pressure sensor and determines the pressure in the common rail by look-up using a map listing an experimentally obtained relation between the pressure of fuel within the common rail and the output voltage of the fuel pressure sensor. The electronic control unit controls the quantity of fuel supplied to the common rail to bring the determined pressure in the common rail into agreement with a target value under feedback control.
The voltage applied to the sensor element may drop below the power supply voltage produced by the power supply circuit due to addition of a resistor in the harness connecting between the fuel pressure sensor and the electronic control unit in order to increase an output of the engine or disconnection of the harness from the fuel pressure sensor or the electronic control unit. This will cause the voltage, as outputted by the fuel pressure sensor, to have a level lower than that indicating an actual value of the pressure in the common rail. The electronic control unit, thus, computes the value of the pressure in the common rail in error as being smaller than an actual value thereof. This causes an excessive quantity of fuel to be fed from the high-pressure pump to the common rail, so that the pressure of fuel within the common rail is elevated above the target value, thus resulting in an undesirable rise in output of the engine, which may lead to physical damage to or a decrease in service life of the common rail.